Air is separated in air separation plants that employ cryogenic rectification to separate the air into products that include nitrogen, oxygen and argon. In such plants, the air is compressed, purified of higher boiling contaminants such as carbon dioxide and water, cooled to a temperature suitable for the distillation of the air and then introduced into a distillation column system.
In one typical distillation column system, the air is separated in a higher pressure column into a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms, also known as kettle liquid. A stream of the crude liquid oxygen column bottoms is introduced into a lower pressure column for further refinement into an oxygen-rich liquid column bottoms and a nitrogen-rich vapor column overhead. The lower pressure column operates at a lower pressure than the higher pressure column and is thermally linked to the higher pressure column by a heat exchanger known as a condenser reboiler. The condenser reboiler condenses a stream of the of the nitrogen-rich vapor column overhead through indirect heat exchange with the oxygen-rich liquid column bottoms to produce liquid nitrogen reflux for both the higher and lower pressure columns and to create boilup in the lower pressure column by vaporization of part of the oxygen-rich liquid column bottoms produced in such column.
In any type of air separation plant, liquid and vapor that can be composed of nitrogen-rich and oxygen-rich liquid and vapor are introduced into a main heat exchanger and passed in indirect heat exchange with the incoming air to help cool the air and to be taken as products from the warm end of the main heat exchanger. In addition, liquid products enriched in oxygen, nitrogen or both can be taken from the distillation column system as liquid products. Also, all or a portion of liquid streams removed from columns can be pumped to produce a pumped or pressurized liquid which is heated in the main heat exchanger or a separate heat exchanger designed to operate at high pressure and produce a enriched products as either a vapor or a supercritical fluid.
Since an air separation plant must be maintained at cryogenic temperatures in order to allow the air to be distilled, refrigeration must be imparted to the plant in order to compensate for heat leakage into the plant and warm end losses from the main heat exchanger or other heat exchanger operated in association therewith. Further, the removal of liquid products will also remove imparted refrigeration that must also be compensated through introduction of refrigeration into the plant. This is commonly done by forming a compressed refrigerant air stream by introducing the compressed and purified air into a booster compressor. The compressed refrigerant air stream after such further compression is then introduced, either directly or after partially cooling such stream, into a turboexpander to produce an exhaust stream that is introduced into the distillation column system. In this regard, such exhaust stream can be introduced into the lower pressure column or the higher pressure column.
In large part, the ongoing expense in operating an air separation plant is the cost of electricity that is consumed in compressing the air. As mentioned above, when liquid is to be taken as a product, further compression will be required to generate the refrigeration that will be required when such products are produced. However, the demand for liquid products and the cost of electricity are not constant. For instance, the cost of electricity and the liquid demand will often be less during evening hours as compared with daylight electricity costs and liquid demands. Consequently, air separation plants can be designed to cyclically produce a greater share of liquid products when electricity is less expensive, store such liquid products and then reduce the production of liquid during daylight hours.
Air separation plants that are designed to be able to produce liquid products at both high and low rates of liquid production are well known in the art. Generally speaking, such plants employ a bypass line that bypasses the booster compressor. When it is desired to produce liquid products at a lower rate, valves route the flow that would otherwise be introduced into the booster compressor in the bypass line. The bypassing of the booster compressor will decrease the pressure ratio across the turboexpander and therefore, the amount of refrigeration able to be imparted to the air separation plant.
However, not all of such plants are able to be cyclically operated between high and low rates of production. For instance, U.S. Pat. No. 5,901,579 discloses a system in which a turbine booster can be bypassed to in turn decrease the pressure ratio across a turbine. However, the arrangement shown in this patent is not capable of being operated in a fashion in which liquid production would be cycled between high and low liquid makes. Such a system can either be set in a high mode of production in which a booster compressor is used or a low mode of production where the booster compressor is bypassed and not used. In the flow circuit shown in this patent, if the booster compressor were bypassed without turning off the plant, the booster compressor would immediately go into surge. Surge, as well known in the art, is a damaging oscillating flow condition within the compressor that is brought about by exceeding the pressure ratio at a specific compressor speed. Also, it would not be possible to gradually take the booster compressor off line because in the flow circuit shown in this patent, the compressed air would reverse its direction and flow into the pre-purification unit.
Even in plants that are designed to be cyclically operated between high and low rates of liquid production, the range of range of liquid production that is able to be realized is very limited. One major reason for this has to do with surge. In order to avoid surge, the bypass line itself is utilized to initially recycle compressed air from the outlet of the compressor to the inlet of the booster compressor when it is brought on line or is taken off line or into a low pressure operating mode. The problem with this is that a valve in the bypass line is used for such purposes and unless the booster compressor has only limited compression capabilities as compared with a plant operation using the bypass, flow to the turboexpander could be disrupted leading to the turboexpander being damaged. Moreover, often the turboexpanders used in such plants are directly coupled to a compressor on a common pinion in an arrangement known as a booster loaded turbine. When pressure is increased by the booster compressor, the speed of the turboexpander and therefore, the coupled compressor will increase to drive the compressor toward surge. As can be appreciated this also limits the compression capabilities of the booster compressor and consequently the variation of pressure ratio that can be applied to the turboexpander. As a result, in such arrangements the degree to which the air separation plant can be turned down to decrease liquid production is very limited. Consequently, the power savings of such a plant during periods at which low liquid production rates are desired is also limited.
As will be discussed, the present invention provides a method of separating air and an air separation plant which among other advantages, allows a booster to by bypassed to turn down liquid production with greater liquid turndown capabilities than are contemplated in the prior art.